The distribution of small electronic devices such as smartphones and tablet personal computers has now made it common for a user to carry and use these small electronic devices outdoors for a long period of time. As a result, a battery, which is a power supply for those small electronic devices, is required to have a high capacity so as to be capable of withstanding the prolonged use. Active research and development is underway regarding a lithium ion secondary battery satisfying the above-described requirement. At the same time, an effort is being made to improve the function and performance of small electronic devices such as smartphones and tablet personal computers, and, in such high-function and high-performance small electronic devices, an increase in power consumption is unavoidable. Therefore, the requirement for increasing the capacity of the battery is gradually intensifying.
In addition, recently, in response to the growing awareness of the crisis in energy supply and demand or environment-oriented consciousness, an increasing number of independent distributive power generation facilities such as wind power generation, mega solar power generation, and domestic solar power generation, which are different from conventional centralized power plants, have been installed. However, the problem of power generation facilities using natural energy such as wind power generation and solar power generation, which is the inferior stability of electric supply to that of the conventional power generation facilities, has not yet been solved. Ever since the deterioration of the power feeding status resulting from the Great East Japan Earthquake occurring on Mar. 11, 2011 and the subsequent nuclear power plant shut down, the importance of securing power at individual plants and houses in case of the occurrence of disasters such as earthquakes has been widely recognized. Therefore, a stationary storage battery enabling the securement of a power supply at individual power-consuming places has been attracting attention. However, according to the current techniques, an extremely large storage facility is required in order to secure electric capacity using the stationary storage battery. Therefore, at the moment, such a storage facility lacks practicality when the residential environment in Japan is taken into account.
Furthermore, in the car industry, an electric vehicle and a hybrid vehicle having favorable energy efficiency have been attracting attention, and active development is underway regarding these vehicles. However, the problems of the insufficient cruising distance resulting from the insufficient battery capacity and the absolute lack of charging facilities in towns have not yet been solved. Therefore, at the moment, electric vehicles relying only on electrical energy as an energy source have not become as widely distributed as hybrid vehicles.
A common product that supports the above-described industries such as electronic devices, power securement, and vehicles is a lithium ion battery, and a common cause of the above-described problems is the lack of capacity per unit volume of the lithium ion battery. A significant cause of the problem of the lack of capacity per unit volume of the lithium ion battery is that the discharge capacity per unit volume of a positive electrode active material used for the lithium ion secondary battery is small.
As the positive electrode active material for the lithium ion battery, a cobalt-based positive electrode active material represented by lithium cobalt oxide (LCO) has been used. When an electrode is produced using lithium cobalt oxide, it is possible to achieve a very large electrode density of greater than 3.9 g per cubic centimeter. However, on the other hand, the discharge capacity of lithium cobalt oxide is as small as approximately 150 mAh/g.
As the positive electrode active material for a lithium ion battery, a nickel-based positive electrode active material represented by LNCO (a composite oxide of Li, Ni, and Co), particularly, LNCAO (a composite oxide of Li, Ni, Co, and Al) is also being studied. The discharge capacity per unit weight of LNCAO is greater than that of cobalt-based positive electrode active materials and exceeds 190 mAhg−1. However, this active material has a low density, and it is difficult to increase the electrode density, and thus it has not been possible to improve the discharge capacity per unit volume.
Patent Documents 1, 2, and 3 describe that the discharge capacity per unit volume of a lithium ion battery and the discharge capacity-holding properties are related to the breakdown strength or pressurized density of the positive electrode active material. Patent Document 1 describes the adjustment of the breakdown strength of the active material by controlling the composition and average particle diameter of an LCO-based positive electrode active material. Patent Document 2 describes the adjustment of the compressive strength of an LNCO-type positive electrode active material obtained by controlling the quantitative ratio between Ni atoms and Co atoms and powder characteristics of a Ni—Co hydroxide which is a raw material of the positive electrode active material. Patent Document 3 describes the adjustment of the pressurized density of an LNMCO-type positive electrode active material using a special spray drying method in the production of the active material. However, in this prior art, an LNCAO-type nickel-based positive electrode active material has not been studied.